The Design of Modern Steel Bridges - TEDI

68 The Design of Modern Steel Bridges
the horizontal wind pressure given by the new loading is 21 lb/ft 2 , which can be
compared with the values of 23 and 19.5 lb/ft 2 given by the British Standard for
mean hourly wind speed of 64 mile/h, 10 m height and bridge lengths 60 m and
200 m, respectively. The proposed American drag coefficients of 1.5 and 2.3
for solid and truss girders may be compared with the British values of 1.4 to
1.0 for solid girders with b/d ratios ranging from 4 to 12, and 1.6 on the
windward truss and 0.7 on the leeward truss with flat-sided members, with a
solidity ratio of 0.5 and a shielding factor of 0.5. The lift coefficient of 1.0 for
vertical wind load in the American proposals can be compared with the British
value of 0.75 for superelevation between 1 and 5 degrees. With traffic present
on the bridge the American proposal specifies V 30 ¼ 55 mile/h, which is
equivalent to a gust speed of 1.41 0.8 55 ¼ 62 mile/h or 27.6 m/s, but with
the wind load dependent upon the height of the bridge deck above ground or
water level; this may be compared with the gust speed of 35 m/s for all heights
stipulated in the British code.
In the German code[6], wind load is specified as 2.5 kN/m 2 without traffic,
and 1.25 kN/m 2 with traffic, to be applied to the area in projected elevation of
the bridge. The traffic profile is taken as a 2 m high vertical surface above the
bridge deck. The German loading[10] retains the above wind loading for
bridges with superstructure 50 to 100 m above ground level, but makes
reductions for:
(1) superstructures at lower height
(2) superstructures with noise barriers, in the load case without traffic.
It also increases the height of the traffic profile to 3.5 m.
3.7 Thermal forces
If the free expansion or contraction of a structure due to changes in temperature
is restrained by its form of construction (e.g. portal frame, arch) or by
bearings or piers, then stresses are set up inside the structure. Secondly, differences
in temperature through the depth of the superstructure set up stresses
if the structure is not free to deform. A differential temperature pattern in the
depth of the structure represented by a single continuous straight line from the
top to the bottom surface does not cause stresses in a statically determinate
structure, e.g. simply supported or balanced cantilever spans, but will cause
stresses in a continuous structure due to the vertical restraints provided by the
piers. Normally differential temperature is not represented by a single continuous
line from the top to the bottom surface, and hence causes stresses even
in simple spans.
In the British Standard BS 5400[2], maps of isotherms provide the extremes
of shade air temperatures at sea level in different parts of the British Isles. For
heights above sea level these temperatures are reduced by 0.5 C and 1.0 C for